Water, wastewater and industrial liquid granular filtration units typically have a filter media support system that separates the filter media from the underdrain system and filter bottom. The underdrain system is the primary support for the filter media, and also serves to collect the filtrate and provide for the uniform distribution of air and water during the backwash of the filter system. Because high pressure air and water backwash are an important part of the filter system, underdrain blocks must be heavy enough so as not to move or shift during backwash. Typically, underdrain blocks have been precast from high strength concrete and the blocks set in parallel rows in the bottom of the filter to provide support for the filter media and protection of the filter infrastructure of air laterals.
Underdrain systems are often made of concrete blocks having spaces to allow for piping, such as air laterals, that are part of the backwash air distribution system. A solid precast concrete, plastic-jacketed underdrain block is disclosed in U.S. Pat. No. 4,923,606 to Gresh. In the Gresh apparatus, the plastic jacket functions as a mold for concrete. French foreign reference 544,011 discloses a filter support surface support comprised of molded concrete slabs formed with a number of closely spaced fine holes to allow air and water through.
Brown, U.S. Pat. No. 5,149,427 discloses a cap for a filter underdrain block. The cap is a porous planar body of sintered polyethylene having pores in the range of 700–800 microns. The underdrain block comprises interior walls that define three conduits. In Brown U.S. Pat. No. 5,232,592, a continuation of Brown '427, discloses that the cap and the ribs on the top of the underdrain block define a plurality of distribution chambers above the orifices in the block.
Brown, U.S. Pat. No. 5,269,920, is also a continuation of the '427 reference. The top surface 12 of the cap includes a plurality of tapered, spaced screen members 20. Each screen member 20 defines a plurality of slots in the top surface 12. The slots are sized to provide fluid communication between the filter media and the distributor without passage of fine grain media therethrough.
U.S. Pat. No. 5,108,627 was given to Berkebile for a filter underdrain block. The '627 patent discloses a multi-chambered block having separate conduits for gas and liquids for a backwash cycle. The '627 reference also teaches that the block can be formed from molded, high-density polyethylene.
None of the underdrain blocks available prior to the present invention teach or suggest a block that can be weighted with a variety of materials that are available on site and sealed. What is needed is an underdrain block with a nonporous lid that allows the block to be filled with a multitude of materials that are on hand at the filter site to give greater mass to the underdrain block.